Orthogonal frequency division multiplexing (OFDM) has gained considerable interest in recent years. In an OFDM system, data are modulated on frequency domain subcarriers. The modulated subcarriers are degraded when passing through a multipath channel. A channel is defined as the medium between a transmitter and a receiver, and multipath results in radio signals reaching a receiver by two or more paths. The time delay caused by the multipath effect is called echo, and a long echo is a received signal having a long time delay. Due to the multipath effect, subcarriers will suffer degradation which must be corrected at the receiver.
For coherent detection, these subcarrier frequency responses must be estimated through the use of pilot tones. For this reason, pilot tones are inserted in OFDM symbols to enable the interpolation of the channel response. Pilot tones are usually distributed in both frequency and time dimensions to enable the estimation of time-varying channel response.
FIG. 1 shows a sequence of OFDM symbols, wherein data and pilot tones are modulated into K subcarriers. In the example shown, K is 1704 and 6816 in a 2K-mode and an 8K-mode DVB-T system, respectively. The remaining subcarriers are the empty subcarriers at both ends of the frequency band. Solid black circles represent pilot tones while white circles represent data subcarriers transmitted from a DVB-T transmitter station. Guard intervals preceding each OFDM symbol are not shown.
In DVB-T receivers, the channel estimation is carried out through pilot signal estimation, and the accuracy of the interpolation depends on the number of pilots present in OFDM symbols.